It is known in principle that rechargeable cells and, in particular, electrochemical energy storage cells, such as rechargeable batteries for example, comprise two electrodes, and in particular one positive electrode or positively charged electrode, such as the anode for example, and one negative electrode or negatively charged electrode, such as the cathode for example, a chemical reaction taking place between said electrodes. Electrical energy is released on the basis of this chemical reaction. In the case of a rechargeable nickel-cadmium battery for example, the positive electrode comprises nickel compounds, such as nickel hydroxides for example, while the negative electrode comprises, for example, cadmium. Accordingly, the negative electrode of a rechargeable nickel-hydride battery comprises, for example, a hydrogen-storing metal alloy. As is known in principle, rechargeable lithium-ion batteries in particular are used very frequently, wherein the positive electrode of said rechargeable lithium-ion batteries comprises a lithium metal oxide, while the negative electrode advantageously comprises special carbons which can incorporate lithium ions. It is furthermore considered to be known in principle that the individual electrodes are spaced apart and, in particular, insulated from one another by means of a separator, in order to prevent an internal short circuit and consequently to prevent the electrical energy being lost in the form of heat. Furthermore, an electrolyte is required in particular, in order to allow an electrochemical reaction to take place, wherein the electrolyte itself can be, for example, a liquid which contains conductive salts. The electrolyte itself is located, for example, in the electrodes and the separator, wherein, in the case of rechargeable nickel batteries, potassium hydroxide solution or, in the case of rechargeable lithium-ion batteries, lithium salts which are dissolved in special organic solvents are used in particular. In addition to an electrode winding, the electrochemical energy storage cell can also have a housing which comprises, for example, a cell vessel and a cell cover. Therefore, it is to be considered to be known in principle that a cell vessel of a housing of this type can also serve, for example, as a negative outgoing conductor, that is to say the negative pole, while the cell cover can be used as the positive outgoing conductor, that is to say the positive pole, for example. Rechargeable cells can be designed in the form of winding cells in which at least the positive electrode, the separator and the negative electrode are placed one above the other and wound up, for example in the form of a strip-like sandwich, in such a way that the electrode forms a surface which is as large as possible, this, in turn, being advantageous in respect of short charging times and a high level of performance of the rechargeable battery.
One indicator of the state of a battery and, in particular, of a rechargeable battery which has at least one electrochemical energy storage cell and advantageously a plurality of electrochemical energy storage cells and which can be used multiple times both in stationary applications, such as in wind power plants for example, or in vehicles, such as in hybrid or electric vehicles for example, and also in the consumer sector, such as in laptops or mobile telephones for example, is the state of charge. Said state of charge is of particular importance since very stringent requirements are imposed on the battery systems or rechargeable battery systems used in respect of the reliability, performance and service life of said battery systems or rechargeable battery systems. The state of charge of an electrochemical energy storage cell and, in particular, of a galvanic cell of the rechargeable battery, which is also called the SOC, is determined by means of a cell voltage to the extent that it is known in principle. This means that, in particular, the open circuit voltage (OCV) is measured in the inoperative state. However, in this case, only the external cell voltage is recorded, and therefore only the total voltage of the cell, and in particular of the energy storage cell, can consequently disadvantageously be measured, but the potentials of the individual electrodes of the cell, and in particular of the energy storage cell, cannot be determined.